Vol 58, No 6 (2017)

A brief review of the results of studying some classes of nitrogen-containing chelate boron complexes by ultraviolet photoelectron spectroscopy and density functional theory is reported. The quantum chemical modeling of the substitution effects of a complexing agent, heteroatoms, and functional groups in α, β, and γ positions of the chelate ring allowed us to establish the features of the electronic structure of the studied complexes. It is found that the substitution of heteroatoms in the chelate ring has no substantial influence on the structure of the highest occupied molecular orbital (HOMO). In imidoylamidinate complexes, as opposed to formazanates and β-diketonates, there is no noticeable mixing of π orbitals of the chelate and benzene rings. In condensed nitrogen heterocycles the HOMO is stabilized by 0.2-0.3 eV and π orbitals of the benzene ring are stabilized by 0.8-1.2 eV. The HOMO of substituted aza-boron-dipyridomethene correlates with anthracene and acridine π₇ orbitals, which causes the fine structure of the first band. It is shown that in an energy range below 11 eV the calculated results reproduce well the energy gaps between the ionization states of the complexes.

The article examines the electronic structure and orbital nature of luminescence excitation in a series of molecular crystals with the general formula E_nAX₆, where E_n are organic and inorganic cations (diphenylguanidinium, guanidinium, and cesium); n is the number of cations; AX₆ are Te(IV) and Sb(III) anions; X are the atoms of halogens Cl or Br. The electronic structure of these molecular crystals is determined from the data of X–ray photoelectron spectroscopy of the core and valence levels and еру quantum chemical modeling фе the density functional theory level together with the previously obtained single crystal X–ray diffraction data.

By UPS spectroscopy of vapor, XPS spectroscopy of the condensed phase, and quantum chemical methods the adducts of tris-β-diketonates Eu(асас)₃Phen and Eu(hfас)₃Phen are studied. The electronic structure and features of the nature of chemical bonds in the adducts are established. The geometric structure of the studied compounds in the gas phase is determined. A procedure is developed that helps to assign the bands in gas-phase HeI photoelectron spectra and also in the valence band of the XPS spectra. Quantum chemical calculations (DFT) make it possible to find the regular changes in the electronic structure of the chelate complexes depending on ligand fluorination, to study the effect of a 1,10-phenanthroline molecule on the electronic structure of the chelate rings, and also to analyze the electronic effects caused by trifluoromethyl substitution for methyl groups in the ligand.

A series of Ni- and/or Cu-containing coatings formed by plasma electrolytic oxidation on aluminum and titanium are examined by X-ray photoelectron spectroscopy. Binding energies of core electrons, elemental composition, chemical state of elements, and features of the structural organization of the surface and nearsurface layers of the coatings are determined. A combination of the data collected indicates similar regularities of the composition and significant distinctions in the structure of the coatings formed. It is shown that the coatings formed on titanium are characterized by a considerably higher phosphorus concentration, and correspondingly, phosphates, unlike the coatings formed on aluminum, in which base metal and 3d element (Ni or Cu) oxides are dominant. In both cases, Cu is mainly concentrated in the surface layers of the coatings whereas Ni is mainly concentrated in the near-surface layers.

XPS, PbL₃ and CuK EXAFS, solid state NMR, and EPR techniques are used to study insoluble products formed in the interaction of aqueous solutions of lead(II) nitrate and copper (II) sulfate with potassium nbutylxanthate (KX). The XPS spectra of lead xanthates with the composition PbX₂ are similar to those of KX, and interatomic distances of 0.279 nm suggest a nearly ionic character of Pb–S bonds. In copper xanthate precipitating together with dixanthogen (approximately 15 wt.%), the Cu(I)–S bond length is smaller (0.229 nm), and copper coordination number of 2.9 in a composite with dixanthogen increases to 3.3 after its removal by washing with acetone. The XPS spectra indicate the covalent character of the bond and non-equivalence of xanthate radicals. Solid state ¹H and ¹³C NMR spectra as well as the actual absence of metal lines under the measurement conditions demonstrate strong disordering of the structure of xanthates, which is stronger for PbX₂ and weakest in CuX after the removal of dixanthogen. EPR reveals sulfur-containing radicals and Cu²⁺ in CuX, however, their amounts are insignificant and decrease after the washing with acetone. The results of the work are significant for the understanding of the reactivity of xanthates, in particular, under the conditions of flotation of base metal ores.

The regularities of the electron energy dissipation found in the subsurface atomic layer are valid in the bulk of a solid, too. On the example of model graphite-based materials it is shown that energy losses in X-ray photoelectron spectra agree with the calculated valence electron excitation spectra in analogous unit cells. The control of conjugate electron transitions opens the way to gain new data on the geometry, character, and order of bonding between atoms in the sample by the conventional electron spectroscopy and quantum chemistry methods.

X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy are used for in situ studies of the electronic structure of lithiated natural graphite produced by thermal deposition of lithium upon graphite in a vacuum. By XPS and NEXAFS spectroscopy it is found that lithium vapor thermal deposition results in the formation of a lithiated graphite surface layer and a change in its electronic structure. Based on the quantum chemical simulation of the experimental СK_α XES spectrum of lithiated graphite, it is found that lithium atoms are located mostly on the edges of graphite crystallites. Atomic force microscopy reveals that the size of natural graphite flakes varies from 50 nm to 200 nm.

The functional composition and electrochemical behavior of the samples of N121 oxidized nanosized technical carbon in aqueous electrolytes are studied. For oxidation a 30% aqueous hydrogen peroxide (H₂O₂) solution and 2% H₂O₂ with the addition of singlet oxygen or ozone were used. By means of X-ray photoelectron spectroscopy data and the analysis of the near edge X-ray absorption fine structure the features of the chemical structure of the samples are found. The oxygen concentration did not exceed 5 at.% in the samples. The analysis of cyclic voltammograms reveals that at low scan rates the specific capacitance of the material is determined by the functional composition of the surface. The sample oxidized by 30% H₂O₂ solution and containing the largest number of–OH and–COOH groups demonstrated the highest capacitance in 6M KOH and in 1М H₂SO₄ it was the sample with the highest concentration of C=O groups formed during the oxidation with singlet oxygen. The stability of carbon electrodes is studied in supercapacitor models.

Single-wall carbon carcass nanoparticles (nanohorns) (CNHs) are synthesized by two methods: electric arc and electron evaporation of graphite in the inert atmosphere. Distinctions in the structures of obtained materials are revealed using electron microscopy and Raman spectroscopy. The chemical structure of the CNH surface is investigated by X-ray photoelectron and NEXAFS spectroscopy during oxidation. It is found that oxidation causes the destruction of CNH agglomerates and weakly affects the structure of graphene nets. However, these changes are sufficient for an increase in the infrared radiation absorption by the dispersion of nanohorns in water. It is shown that the efficiency of 808 nm laser heating of a CNH dispersion depends on the synthesis method and chemical modification of nanoparticles, which enables their potential use for local hyperthermia of cells of living organisms in cancer therapy.

The electronic energy structures of three phosphorus-containing sulfides are studied from the experimental K and L_2,3 X-ray emission spectra, K absorption spectra of sulfur and phosphorus, X-ray photoelectron spectra, and also quantum chemical calculations based on density functional theory (DFT). The fullpotential and all-electron quantum chemical calculations are carried out using the LAPW+lo basis set implemented in the WIEN2k software package [1]. The following exchange-correlation potentials are used for the calculations: PBE, PBE+U, and mBJ [2]. The spin-orbit coupling of Tl 5d_3/2 and 5d_5/2 electronic states are taken into account in Tl₃PS₄. All the specific features of the electronic energy structures of the compounds under study are determined in the valence and conduction bands near the Fermi level. The obtained band gaps E_g are in good agreement with the literature experimental data.

Nanocomposite systems based on ternary ZnS_xSe_1–x semiconductor compounds with different compositions (x = 0, 0.3, 0.5, 0.7, 1) in dielectric matrices of nanoporous anodic aluminium oxide (AAO) are synthesized by high vacuum thermal evaporation of a zinc sulfide and selenide powder mixture. The effect of the atomic concentration of solid solutions and the structural parameters of the AAO template matrix on the crystal structure of the synthesized nanocomposites and the local atomic environment of Zn and Se atoms is investigated.

In the current research, the origin of conformational preferences in trifluoro-acetylacetaldehyde (TFAAD) by detail analysis of the intramolecular hydrogen bond (IMHB) and π-electron delocalization (π-ED) was investigated. In this regard, all of the possible conformations of keto and enol tautomers of TFAAD at various theoretical levels were optimized. Our results reveal that the chelated enol forms (E11, E21) have extra stability with respect to the other conformations, and at all of the computational levels E21 is more stable than the E11 and identifies as the global minimum. We expected that the extra stability of E21 is probably related to the stronger intramolecular hydrogen bond (IMHB), but quantum chemical calculations confirm the stronger IMHB of E11. Since, the chelated forms are resonance assisted hydrogen bond (RAHB) systems, it seems that the π-ED concept can probably justify this duality. The significance of π-ED by the geometrical factor of Gilli (λ) and the harmonic oscillator model of aromaticity (HOMA) were assessed. According to these indicators, E21 has higher π-ED than E11, in excellent agreement with their stability order. Finally, the population analysis by the natural bond orbital method and atoms in molecules theory were carried out. The stabilization charge transfer energies of RAHB systems also indicate more π- ED in E21. Consequently the π-electron delocalization effect is the superior factor determining the global minimum. The IMHB of the chelated forms, in the first singlet excited state, were also studied. Surprisingly, the results show that the IMHB in E21 is stronger than in E11, in contrast to the ground state.